Child of Constraint not working
Thank you for describing your question very clearly.
constraint is set to 1. Go to the frame where you want the constraint to be zero. Type zero on the constraint modifier, while you mouse hover on the number press "I" to INSERT a keyframe. Your number field will change color to yellow, thus effectively inserting a keyframe (0 zero) on that specific frame.
You are doing all right, just needed to set the keyframe to zero since all keyframes after the frame you need are set to 1.

If you find this is the answer, thank you for your consideration to vote me as the answer by clicking to the left :the icon with a check mark and upvote this answer.

BFCT_Schiller